I will bring up the point that a lot of the carbon offset programs are questionable in their validity. When offsetting a flight ranges from $4-50, how can we trust the given price? If it can be offset for $4, then why do the predictions for carbon auctions place the price for a ton of carbon markedly higher than the the translation of a $4 price for a plane flight (translating this into tons, it still doesn't add up). This insinuates that either there are some serious arbitrage opportunities in terms of carbon tonnage, or that someone is wrong in their pricing and methodology.

The energy used to pump and treat water in the Las Vegas Valley dwarfs any other type of local electric consumption.

Southern Nevada used about 853.8 million kilowatt-hours of electricity in 2008 to move 439,187 acre-feet of water into valley homes and businesses, according the Southern Nevada Water Authority. Another 119.2 million kilowatt-hours of electricity was used that year to treat 22,501 acre-feet of water and send it back to the lake, according to figures from the Clean Water Coalition, a consortium of local wastewater agencies.

Southern Nevada used 2,107 kilowatt-hours for every acre-foot, or 325,851 gallons, of water delivered, treated and sent back to the lake last year. Nationally, most agencies use between 652 and 6,517 kilowatt-hours per acre-foot, according to Lisa Maddaus, a senior engineer with environmental engineering firm Brown and Caldwell.

Even if carbon offsets are questionable - should you feel guilty about flying (i.e. should you fly?) - I think probably not. Even if the plane is half full and going anyway, your purchase helps boost the economics of that trip. At some point, if enough people decide not to fly, the plane won't go. (the local small airlines has 'mechanical difficulties' every time a flight is woefully underbooked. Then all those people get bumped to the next scheduled flight, some 2 hours later.) Do you really need to fly? Can you call someone or video-conference or skip one trip?

Alright, "should" implies a moral question. The current answers seem to come from what can probably be called a neoclassical capitalist ethic, summed up as informed, individual self-interest. Of course, our self-interest can, like ripples, be extended and extended outward from the individual, but most neoclassical micro doesn't allow for this.

There are also some deep assumptions in your question: the biggest, to me, seems to be that the offset will actually offset the amount of carbon your travel will emit to the atmosphere.

But, maybe you aren't even asking about purchasing an "offset" on the market, but actually offsetting your emission, perhaps by not driving the next week, when you otherwise would have?

However, to answer the simple moral question, without any assumptions about offset markets and the like, then yes, my moral code says I should offset the carbon my actions have put into the atmosphere, given that I know that any additional carbon into the atmosphere right now is a pollutant and a detriment to the Earth. In fact, my moral code says that I should be working to be a net carbon negative to the atmosphere, regardless of others' actions.

I like that you are asking these moral questions on an economics blog.